Energy consumption in grain drying
Why do we dry grain? • The grain moisture content after harveting is in most cases so high that spoiling of grain starts if the grain is not dryed • High moisture content makes possible for the microbes to start their work • Moisture content is the key factor in microbe growth – Low moisture content of material prevents vital functions of microbes – When air moisture content is higher than 62% molds start to grow – When air moisture content is higher than 90% bacterias start to grow
Drying demand in European countries Harvest moisture content and drying need
Harvest moisture %
Germany
Denmark
Sweden
Need for drying %
Finland
100 80 60 40 20 0
Intensity of growth
Microbes and water
0 50
60
70
80
90
Moisture% Sieni
AGTEK410/JA
Bakteeri
100
Crop storage Crop temperarture crop moisture content
Storage time, weeks AGTEK410/JA
• When biomas is kept in a constant air moisture content after some moisture movement between the material and the air there is a balance between the material and air
Material moisture content
Moisture balance
Drying
•Hysteresis makes the moisture balance figures different depending on if the material is drying or wetting • Temperature has an effect on balance, the curves are called moisture isotherms
AGTEK410/JA
Wetting
Air relative humidity %
Moisture balance Material moisture content %
Hay and straw moisture balance 45 40 35 30 25 20 15 10 5 0
Hay Straw Heinä 20 C Heinä 25 C Olki 10 C
Olki 25 C
30
40
50
60
70
Air relative humidity % AGTEK410/JA
80
90
Crop moisture balance Ohra, ruis, vehnä Barley, rye, wheat Tasapainokosteus
30 25
Jyvän kosteus %
Crop moisture content %
Viljat Crops
20 15
Rypsi Rape
10 5 0
15
30
45 60 75 Ilman kosteus Air moisture content %%
Ohra AGTEK410/JA
Ruis
90
Vehnä
Air •Air is a gas mixture •Besides gases air embodies water vapour (moisture) • Air density in normal conditions (1 atm, 0C) is 1.29 kg/m3
AGTEK401/JA
Alkuaine
Tilavuus % Paino %
Happi O 2
20,95
23,14
Typpi N
78,09
75,53
Argon Ar
0,93
1,28
Hiilidioksidi CO 0,03
0,05
Neon Ne
0,001
100 % 90 % 80 % 70 % 60 % 50 % 40 % 30 % 20 % 10 % 0%
0,002
Others Nitrogen Oxygen
Air moisture contents Relative humidity expresses how near air is to saturation, 100 % = saturation
ph s p h s = relative humidity ph = water vapour partial pressue p’h = water vapour partial pressue at saturation
Humidity ratio (absolut humidity) expresses how much the air has water in
mh x mi x
= humidity ratio
mh = mass of water
mi = mass of dry air AGTEK401/JA
Air heating power
ti, hi
tp, hp
Pk Vi ci T ci ΔT
air heat capacity, n 1,006 kJ/kgK temperature change
This is an approximate way to calculate air heating power AGTEK401/JA
Example Grain dryer furnace air flow value is 21 000 m3/h and the temperature change is 60 C. What is the heating power of the furnace?
AGTEK401/JA
Grain drying In boreal conditions crops must be dryed. Drying consumes enerfgy!
Dry grain out
Humid air out
Ambient air in
Oil and electricity in
In grain drying the moisture in the grain moves to the air, which is blown through the grains
Drying During drying different phases can be seen • Drying starts with rapid moisture removal, the free water on the grain surface is removed • When the free water has been removed water inside the grain must move to the surface, the moisture removal speed is decreased • Temperature has an effect of water moving speed, high temperatures give higher moving speeds AGTEK410/JA
Drying Grain moisture content Grain core temperature Constant speed
Decreasing speed
Drying speed Drying time AGTEK410/JA
Drying Removed water amount:
Example • Barley yield of Finland is 1 700 milj. kg. What is the evaporated water amount when harest moisture content is 25, 20 or 18%.
Drying energy need • The minimum energy need is got from the water evaporation energy and vapour heat content E hvh cvTv hvh = water evaporation energy at 0 C, 2502 kJ/kg cv
AGTEK410/JA
= water vapour heat content, 1,87 kJ/kg C
Example • What is the minimum energy needed to remove 1 kg of water at 50 C?
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Drying In grain drying • 120 g of burning oil is needed per evaporated water kg • This corresponds to 5,1 MJ/kg H2O
• Besides this energy (electricity) is needed to run the fan, elevator and other dryer equipments
Example • A farm has 210 ha of grain with average yield of 3,5 t. What is the average oil consumption in grain drying?
This material has been produced in ENPOS project. ENPOS is acronym for Energy Positive Farm. The project partners are • University of Helsinki, department of Agricultural Sciences – Agrotechnology • MTT Agrifood Research Finland - Agricultural Engineering • Estonian University of Life Sciences Project home page is at http://enpos.weebly.com/ The project is financed by the EU Central Baltic IV A Programme 2007-2013 This publication reflects the authors views and the Managing Authority cannot be held liable for the information published by the project partners.
